Among other things, skeletal structures in human beings support the weight of an individual, active or resting. When a person is seated on a surface wide enough to support them, the left and right ischium components of their pelvis are the parts of the skeletal structure designed to transfer the load (weight) of their upper body to the surface they are seated on.
Motorcycle seats are purpose built. They allow for maximum rider agility while riding.
These seats can be forced up in between the ischium structures of the pelvis of some seated riders. When seated, these people now have only soft tissue supporting any weight not supported by their hands or feet. This quickly leads to discomfort as the muscles, organs, and vascular structures normally protected by the bones of the pelvis are asked to do something they are not designed to do, compress to support loads.